This invention relates to a highly cross-linked polyurethane aqueous dispersions which produce coatings having excellent properties such as fastness to light. The invention also relates to a method of preparing such dispersions which are characterized by excellent shelf life.
Numerous examples are known for the preparation of aqueous polyurethane dispersions.
These are very largely so-called amine-lengthened polyurethane-urea systems of linear structure with, in some cases, variable molecule segment pairing.
Systems of this kind are efficient, particularly in the field of highly flexible substrate coating, for example substrates of textile or similar materials.
For coating of a considerably larger number of other substrates, such as wood, leather, cardboard, metal, concrete, synthetic leather, etc., with aqueous varnish dispersions, the films produced should have a sufficiently high block temperature adapted to technical requirements. Dispersions of polyurethane-urea prepolymers of a strictly linear structure exhibit practically only block or deformation temperatures below 100.degree.C.
The more pronounced the thermoplastic character of a film of varnish even in the range below 100.degree.C, the more clearly will this property result in liability to soiling, adhesion on stacking, and susceptibility to solvents, to mention only a few limitations in application.
Dispersion coatings subjected to the direct irradiation of the sun moreover naturally require great stability to light, and if possible, also a high block temperature.
It was therefore desirable to produce polyurethane dispersions of this kind, based on highly cross-linked polyurethane-prepolymer systems, which could comply with these technical requirements.
The difficulty of preparing dispersions of highly-cross-linked polyurethane prepolymers for the production of highly thermo-stable coatings is sufficiently known. Numerous experiments with appropriate diisocyanates have been unsuccessful because of the sedimentation of the resulting dispersions either during their actual production, when further processed, or when stored for a short time.